1. Field of the Invention
The present invention relates to a feedback control scheme for a plant. More particularly, the present invention relates to control of a variable lift system, control of a variable phase system, and air-fuel ratio control of an internal combustion engine.
2. Description of the Related Art
When a plant has a strong non-linear property, a general linear feedback controller such as PD and PID has problems in following ability and stability, and thus hardly realizes high-precision control. For example, high-precision feedback control is hard to achieve for a variable lift system of an internal combustion engine because it has a large friction and has a non-linear property such as a hysteresis property relative to increase/decrease of lift amount. Similarly, a variable phase system and/or an air-fuel ratio control system for an internal combustion engine and an actuator control system for an automatic transmission have a strong non-linearity.
Control of an internal combustion engine is based on realization of highly precise operations of a plurality of components. High precision is required as to operation stability and following ability for such components with a strong non-linear property mentioned above. Accordingly, a control technique applicable to plants having a strong non-linear property is needed.
As a control method for compensating for non-linear property of a plant, sliding mode control with two-degree-of-freedom has been proposed (see Patent Document 1, for example). When controlling a controlled object that has a non-linear property such as friction and hysteresis property, sliding mode control with two-degree-of-freedom compensates for the non-linear property by introducing non-linear input capable of controlling output of the controlled object to a target value with high precision and high response. As the method can specify an error convergence property separately in terms of responsiveness of following a target value and disturbance, it exhibits excellent overshoot suppression capability when the target value is changed.
Patent Document 2 discloses a control method that adds dither input to a sliding mode controller. This method uses dither input to correct a control amount that is produced from the sliding mode controller for feedback-controlling of a plant to a target value. This process compensates for degradation of controllability due to a non-linear property of a plant such as a friction property.
[Patent Document]: Japanese Patent Application Publication (JPAP) No. 2005-11036
[Patent Document]: JPAP No. 2001-152885